Is it possible to secure major events?

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Terror attack disrupts Boston Marathon

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"We have reviewed and will continue to review our security arrangements with the Metropolitan Police and other authorities," a statement on the event website said Tuesday.

Chief executive Nick Bitel said: "We want to reassure our runners, spectators, volunteers and everyone connected with the event that we are doing everything to ensure their safety."

The organizers said Monday they were deeply saddened and shocked by the news from Boston. "Our immediate thoughts are with all the people there and their families. It is a very sad day for athletics and for our friends and colleagues in marathon running," they said in a statement.

London is the next, after Boston, of the six races that make up the World Marathon Majors series.

The course, which starts in southeast London, passes through some of the capital's main business districts before finishing near Buckingham Palace.

More than three-quarters of those taking part will raise money for charity.

'Robust security measures'

"The bombings in Boston are shocking, cowardly and horrific, and the thoughts of all Londoners this morning will be with the victims," London Mayor Boris Johnson said in a statement Tuesday.

"Boston is a proud city built on history, tradition and a real sense of community. These attacks were aimed at its core, at innocent men, women and children enjoying a spring day out at a major sporting event.

"We do have robust security measures in place for Sunday's London Marathon, but given events in Boston it's only prudent for the police and the organizers of Sunday's race to reexamine those security arrangements."

CNN anchor Piers Morgan tweeted: "London marathon this Sunday - security at these events will never be the same again."

Will Geddes, managing director of threat management company International Corporate Protection, told CNN that it is "very difficult" to secure a marathon.

"You can look at isolating particular areas and trying to secure these -- however, you are looking at a 26-mile-plus route, which often will spread across a major capital, and in terms of protecting it right along the route, it will be very, very difficult," he said.

Any potential terrorist "will be looking for the largest number of casualties they can achieve, so the start point and the finish point will no doubt be two areas the Metropolitan Police will be focusing on and how they can secure those."

But, Geddes said, "to a certain degree, there is only so much they can do."

One key element will be the awareness of the general public, which in recent years has played an increasing role in alerting authorities to any suspicious activities or bags left unattended, he added.

A big security operation will also swing into place on Wednesday for the funeral of Margaret Thatcher, Britain's first woman prime minister.

The Metropolitan Police have said parts of the transport network will be closed down and there will be a large military and police presence on the streets as the funeral procession passes through central London.

Last summer, authorities successfully implemented a huge security plan to keep the city safe during the London Olympics.